Biosensors that can quantify a specific analyte in a sample simply and rapidly, for example, without dilution or stirring of the sample have been used widely. In general, such a biosensor can be produced by forming an electrode system having a working electrode (also referred to as “measuring electrode”) and a counter electrode on an electrically insulating substrate by a method such as screen printing, and then forming a reactive layer including a redox enzyme that reacts with the analyte and an electron-accepting substance on the electrode system, as disclosed in Japanese Patent No. 2517153, for example. When the reactive layer is in contact with the sample containing the analyte, the analyte is oxidized by the catalytic action of the redox enzyme, and the electron-accepting substance is reduced at the same time. The reduced electron-accepting substance is re-oxidized by an electrochemical approach, and the concentration of the analyte in the sample can be calculated from the thus obtained oxidation current values.
However, such a biosensor has a problem in that there may be errors in the measurement, depending on the properties or the like of the sample. For example, in the case where a whole blood is used as a sample, solid components such as blood cells and soluble/insoluble components such as lipid, protein, and saccharides are adsorbed on the surface of the electrodes, thereby making an accurate measurement difficult. Furthermore, there is a large difference between individuals in the hematocrit (Hct) value, which is a volume ratio of erythrocytes to whole blood, so that there is a difference between specimens in the above-described influence on the biosensor. Such an influence due to the above-described impurities can be decreased by, for example, diluting the sample and then subjecting the diluted sample to a biosensor. However, this takes more time and complicates the operation.
In order to avoid the influence due to the impurities, the following methods have been proposed, for example: forming a water absorptive polymer layer on electrodes (JP 6(1994)-54304 B); forming a layer containing a water insoluble polymer and a water-soluble polymer on a reactive layer (JP 6(1994)-213858 A); forming a polymer film made of a mixture of a lipid-soluble polymer and an amphiphile polymer on electrodes (JP 9(1997)-318588 A); forming an anion filter for suppressing the solid components from passing therethrough on a reactive layer (JP 10(1998)-221293 A); and fixing a microcapsule containing a sodium citrate solution onto an electrode on which glucose oxidase is immobilized, and using the electrode as a working electrode (JP 5(1993) 133929 A).